Rotating gas turbine blades must fulfill a multitude of material and design criteria that consider high mechanical and thermal stresses acting on the rotating blades during operation. Due to enormous centrifugal forces acting onto rotating blades and an enormous thermal load that must withstand the blades, the primary blade design task is to provide a high degree of stiffness and avoid blade vibrations during operation. Active cooling using cooling channels inside the airfoil of a rotating blade must also be considered. Thermal coatings on blades is yet another design consideration. A method for ‘tuning’ the natural frequency of a blade by altering the airfoil shape is sought to improve aeromechanic natural frequency margin to stimuli while maintaining or improving aerodynamic performance.
Rotating blades are arranged in rows which alternate in axial direction with rows of stationary vanes. Every pair of rows include one row of stationary vanes and one row of rotating blades which follows directly downstream to form a so called stage. All stages of the turbine are numbered in sequence beginning with the first stage at the inlet opening of the turbine having the first row of stationary vanes followed by the first row of rotating blades.
Normal operation of a gas turbine shows that the stationary vanes, e.g. of the first stage, are excitation sources for vibrations acting on the subsequent rotating blades resonating at a second natural frequency. Reducing the effects of such excitation sources to avoid vibration transmission and excitation onto rotating blades arranged downstream of vanes in any stage is advantageous. Altering the difference, i.e. separation, between the first natural frequency of the gas turbine blade to the second excitation natural frequency caused by stationary vanes can reduce these effects.
With a conventional airfoil stack, changing airfoil chord distribution is a typical approach to tuning axial frequencies. Typically, root chord is increased and tip chord is reduced in an effort to increase axial frequencies. This will generally increase both the 1st and 2nd harmonics of the axial frequency, so margin to stimuli can only be gained on one (but not both) of these modes. Another approach would be to alter the radial length of the blade, which may require significant changes to the turbine architecture. These changes likely create significant performance penalties.
Current techniques for “tuning” unshrouded blade axial frequencies have several drawbacks. The 1st axial (1A) and 2nd axial (2A) modes tend to respond in a similar manner to airfoil changes, i.e. both increase or both decrease in frequency. Commonly, gaining margin on one mode results in losing margin on the other mode.